Training Optimization of Multiple Lines in a Vectored System Using a Prepared-to-Join Group

ABSTRACT

A novel procedure is described which provides a method for initialization of a group of CPE devices (short: CPEs) during a training that in part registers capabilities of the CPEs, wherein at least one CPE registers late to the training and thus cannot be registered. In accordance with the example described herein, the method comprises: determining capabilities of the CPEs during a Joining Phase of the training, wherein it is determined whether a CPE device is capable of employing vectoring. The method further comprises placing in a hold status the at least one CPE that registers late by keeping a line active that is coupled to the at least one CPE. Another Joining Phase is provided after the Joining Phase in order to register the at least one CPE which has registered late.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/819,578 filed May 5, 2013

TECHNICAL FIELD

The present disclosure relates to VDSL (Very-high-bit-rate digitalsubscriber line) technology, in particular to the initialization of agroup of CPE (customer premise equipment (devices) within a VDSL datatransmission system.

BACKGROUND

Digital subscriber line (xDSL) technology has developed in recent yearsin response to the demand for high-speed Internet access. xDSLtechnology utilizes the communication medium of pre-existing telephonesystems. Thus, both plain old telephone systems (POTS) and xDSL systemsshare a common line for xDSL-compatible customer premises. Similarly,other services such as time compression multiplexing (TCM) integratedservices digital network (ISDN) can also share a common line with xDSLand POTS.

Allocations of wire pairs within telephone cables in accordance withservice requests have typically resulted in a random distribution ofpair utilization with few precise records of actual configurations.Because of the physical proximity of bundled cables (due to pairtwisting, cable branching, cable splicing, etc.), crosstalk caused bythe electromagnetic interference between the neighboring lines is oftenthe dominating noise source in the transmission environment. Inaddition, due to pair twisting in cables where cable branching andsplicing take place, a wire pair can be in close proximity to manydifferent pairs spanning different portions of its length. At atelephone CO (central office), pairs in close proximity may carrydiverse types of service using various modulation schemes, Withconsiderable differences in signal levels (and receiver sensitivities)especially for pairs of considerably different lengths.

There are generally two types of crosstalk mechanisms that arecharacterized, one being far-end crosstalk (FEXT) and the other onebeing near-end crosstalk (NEXT). FEXT refers to electromagnetic couplingthat occurs when the receiver on a disturbed pair is located at the farend of the communication line as the transmitter of a disturbing pair.Self induced far end crosstalk (self-FEXT) generally refers tointerference caused by neighboring lines provisioned for the same typeof service as the affected line, or “victim line.” In contrast, NEXTresults from a disturbing source connected at one end of the wire pairwhich causes interference in the message channel at the same end as thedisturbing source.

Crosstalk (or inter-channel interference) is a major source of channelimpairment for Multiple Input Multiple Output (MIMO) communicationsystems, such as Digital Subscriber Line (DSL) communication systems. Asthe demand for higher data rates increases, DSL systems are evolvingtoward higher frequency bands, wherein crosstalk between neighboringtransmission lines (that is to say, transmission lines that are in closevicinity such as twisted copper pairs in a cable binder) is morepronounced (the higher frequency, the more coupling). A MIMO system canbe described by the following linear model:

Y(f)=H(f)X(f)+Z(f),  (1)

wherein the N-component complex vector X, respectively Y, denotes adiscrete frequency representation of the symbols transmitted over,respectively received from, the N channels, wherein the N×N complexmatrix H is referred to as the channel matrix: the (i,j)-th component ofthe channel matrix H describes how the communication system produces asignal on the i-th channel output in response to a symbol beingtransmitted to the j-th channel input. The diagonal elements of thechannel matrix describe direct channel coupling, and the off-diagonalelements of the channel matrix describe inter-channel coupling (alsoreferred to as the crosstalk coefficients), and wherein the N-componentcomplex vector Z denotes additional noise present over the N channels,such as alien interference, thermal noise and Radio FrequencyInterference (RFI).

Different strategies have been developed to mitigate crosstalk and tomaximize effective throughput, reach and line stability. Thesetechniques are gradually evolving from static or dynamic spectralmanagement techniques to multi-user signal coordination (or vectoring).

One technique for reducing inter-channel interference is joint signalpre-coding: the transmit data symbols are jointly passed through apre-coding matrix before being transmitted over the respectivecommunication channels. The pre-coding matrix is such that theconcatenation of the pre-coder and the communication channel results inlittle or no interference at the receiver. This is achieved by adding tothe original signal an anti-phase signal that is the inverse of anestimate of the aggregate crosstalk signal.

A further technique for reducing inter-channel interference is jointsignal postprocessing: the received data symbols are jointly passedthrough a crosstalk cancellation matrix before being detected. Thecrosstalk cancellation matrix is such that the communication channelresults in little or no interference at the receiver. This is achievedby subtracting from the received signal an estimate of the aggregatecrosstalk signal.

Signal vectoring is typically performed at a traffic aggregation point,whereat all the data symbols that are to be concurrently transmittedand/or received are available. Signal pre-coding is particularly suitedfor downstream communication, while crosstalk cancellation isparticularly suited for upstream communication.

The choice of the vectoring group, that is to say the set ofcommunication lines, the signals of which are jointly processed, israther critical for achieving good crosstalk cancellation performances.Within that group, each communication line is considered as a disturbingline inducing crosstalk into the other communication lines of the group,and the same communication line is considered as a victim line receivingcrosstalk from the other communication lines of the group. Crosstalkfrom lines that do not belong to the vectoring group is treated as aliennoise and is not canceled.

Ideally, the vectoring group should match the whole set of communicationlines that physically and noticeably interact with each other. Yet,limited vectoring capabilities and/or specific network topologies mayprevent such an exhaustive approach, in which case the vectoring groupwould include a sub-set only of all the physically interacting lines,thereby yielding limited crosstalk cancellation performances.

The performance of signal pre-coding and crosstalk cancelling dependscritically on the component values of the pre-coding and cancellationmatrix respectively, which component values are to be computed andupdated according to the actual (and varying) crosstalk couplingfunctions between the respective communication channels.

A known method for estimating the crosstalk coefficients comprises thesteps of:

-   -   simultaneously transmitting a plurality of mutually orthogonal        crosstalk pilot sequences of length L through respective ones of        a plurality of disturber channels,    -   measuring errors induced over a victim channel while the pilot        sequences are being transmitted,    -   correlating the error measurements with respective ones of the        plurality of crosstalk pilot sequences, thereby yielding a        plurality of correlated error measurements,    -   estimating the crosstalk coefficients from the plurality of        disturber channels into the victim channel based on respective        ones of the plurality of correlated error measurements.

That is, transceiver units send mutually orthogonal downstream and/orupstream pilot signals. Error samples, measuring both interference andnoise over the victim channel, are fed back to a Vectoring ControlEntity (VCE). Error samples contain both amplitude and phase informationon a per-tone basis, or on a per-group-of-tones basis. The error samplesare correlated with a given pilot sequence in order to obtain thecrosstalk contribution from a specific line. To reject the crosstalkcontribution from the other lines, i.e. in order to fulfill theorthogonality requirement, a multiple of L error samples shall becollected and processed. The crosstalk estimates are used for updatingthe pre-coding and/or cancellation matrix. The process can be repeatedas needed to obtain more and more accurate estimates.

The orthogonality requirement further implies that the length L of thepilot sequences is lower-bounded by the size of the vectoring group: themore channels, the longer the pilot sequences, the longer the estimationof the crosstalk coefficients.

This known method has been adopted by the InternationalTelecommunication Union (ITU) for use with VDSL2 transceivers, and isdescribed in the recommendation entitled “Self-FEXT Cancellation(vectoring) For Use with VDSL2 Transceivers”, ref. G.993.5 (April 2010).In this recommendation, it is currently envisaged that the pilot signalswould be sent on the so-called SYNC symbols, which occur periodicallyafter every 256 DATA symbols.

On a given disturber line, a representative subset of the activecarriers (or tones) of the SYNC symbol are 4-QAM modulated by the samepilot digit (+1 or −1) from a given pilot sequence, and thus alltransmit one of two complex constellation points, either ‘1+j’corresponding to ‘+1’, or ‘−1−j’ corresponding to ‘−1’. The remainingcarriers of the SYNC symbol keeps on carrying the typical SYNC-FLAG forEOC message acknowledgment. On a given victim line, error samples aremeasured and reported for a specific SYNC symbol to the VCE for furthercrosstalk estimation. In recommendation G.993.5, it is further assumedthat the access node transmits and receives the SYNC symbols over thevectored lines synchronously (super frame alignment) so as pilot signaltransmission and error measurements occur simultaneously.

If a line comes into service (e.g., after modem start-up at subscriberpremises), the crosstalk coefficients from the new joining line into thealready active lines need to be estimated first, and the pre-coderand/or crosstalk canceller be updated accordingly, before the newjoining line can transmit at full power over the DATA symbols, else theraising interference may bring about a line retrain on a few activelines (if the newly induced interference exceeds the configured noisemargin). Similarly, the crosstalk coefficients from the already activelines into the joining line need to be estimated first, and thepre-coder and/or crosstalk canceller be updated accordingly, before thenew joining line starts determining respective carrier bit loadings andgains so as to take full profit from the vectoring gains.

G.993.5 defines new crosstalk acquisition phases during the VDSL2initialization procedure for acquiring the crosstalk coefficients fromthe new joining line into the active lines, and vice-versa.

A first crosstalk acquisition phase is carried out after the HANDSHAKEphase, whereby peer transceiver units acknowledges their mutualpresence, exchange their respective capabilities and agree on a commonmode of operation, and the CHANNEL DISCOVERY phase, during which peertransceiver units exchange basic communication parameters through theSOC channel while transmitting at full power within the assignedcommunication band. The first crosstalk acquisition phase is termedO-P-VECTOR 1 and R-P-VECTOR 1 for downstream and upstream communicationrespectively, and aims at estimating the downstream and upstreamcrosstalk coefficients from the initializing line into the alreadyactive lines. O-P-VECTOR 1 and R-P-VECTOR 1 signals comprise SYNCsymbols only, which are aligned with the SYNC symbols of the activelines, and thus do not impair communication over the active lines.O-P-VECTOR 1 is followed by O-P-VECTOR 1-1; R-P-VECTOR 1 is followed byR-P-VECTOR 1-1 and R-P-VECTOR 1-2.

A second crosstalk acquisition phase is carried out after the CHANNELTRAINING phase takes place, that is to say after the time equalizerand/or the echo canceler have been adjusted, and before the CHANNELANALYSIS AND EXCHANGE phase, that is to say before signal to Noise andInterference Ratio (SNIR) is measured and corresponding bit loading andgain values are determined for the respective carriers. The secondcrosstalk acquisition phase is termed O-P-VECTOR 2-1 and R-P-VECTOR 2for downstream and upstream communication respectively, and aims atestimating the crosstalk coefficients from the already active lines intothe initializing line.

A clause in §10.3 of G.993.5 ITU recommendation states that “if severallines are initialized simultaneously, the initialization procedures ofthese lines have to be aligned in time, so that all lines pass thevectoring-related phases simultaneously (see clauses 10.3.3.6 and10.4.3.9)”. Further in §10.3.3.6 op. cit., the following furthertechnical details are mentioned in case multiple lines are initialized:“The downstream crosstalk channels from the initializing lines into theactive lines of the vector group should be estimated simultaneously byinsuring that O-P-VECTOR 1 signals are sent on all initialization linesduring the estimation. This can be done by controlling the end and thestart of O-P-VECTOR 1 in each line”; and further: “The upstreamcrosstalk channels between the initializing lines and the active linesof the vector group should be estimated simultaneously by insuring thatR-P-VECTOR 1 signals are sent on all initialization lines during theestimation. This can be done by controlling the end of R-P-VECTOR 1 withthe O-P-SYNCHRO V1 signal in each line.”

One option would be to require that lines in a vectoring group arealways activated sequentially. However, this may lead to a denial ofservice for any further lines that want to join after a single line isbeing initialized.

Summarizing the above, FEXT (far-end crosstalk) is the dominant cause ofdisturbances in DMT (discrete multitone transmission) based transmissionsystems such as systems which operate in accordance with the VDSL2standard (see G.993.2, “Very high speed digital subscriber linetransceivers 2 (VDSL2)”). To mitigate FEXT, vectoring has beenstandardized in the VDSL2 standard (see G.993.5, “Self-FEXT cancellation(vectoring) for use with VDSL2 transceivers”). The recommendationG.993.5 covers self-FEXT cancellation in the downstream and upstreamdirections. This recommendation defines a single method of self-FEXTcancellation, in which FEXT generated by a group of near-endtransceivers and interfering with the far-end transceivers of that samegroup is cancelled. The ITU recommendations G993.2 and G.993.5 arehereby incorporated by reference in their entirety.

According to recommendation G.993.5, FEXT is cancelled by the CO(central office) in the direction CPE-to-CO (upstream direction) byestimating the weights of the upstream crosstalk transfer functionsbetween all lines of the cable binder. For any line (referred to asupstream victim line in the following) the receive data of every otherline (referred to as upstream disturber line in the following) withinthe cable binder weighted by its upstream crosstalk transfer function issubtracted from the data received by the upstream victim line. In theopposite direction (downstream), the error containing the FEXT indownstream is estimated by the receiver of a CPE device and transmittedback to the CO where these errors are used to estimate the weights ofthe downstream crosstalk transfer functions between all lines of thecable binder. To mitigate downstream FEXT, the transmit data of any line(referred to as downstream victim line in the following) ispre-distorted by the transmit data of every other line (calleddownstream disturber line in the following) within the cable binderweighted by its downstream crosstalk transfer function. The downstreamsignals are pre-distorted such that FEXT and pre-distortion areneutralized at the receiver of a CPE device.

The weights are estimated in those symbols which are explicitly foreseenfor FEXT estimation and which do not carry any user data. These symbolsare called “sync symbols”. The data carried in these sync symbols mustbe orthogonal from line to line. This orthogonality should not becorrupted in such periods of the training (Training Phase) when thereceived and transmit data is being correlated with the appropriateerror signal. To ensure that added connections do not disturbconnections, which are already exchanging user data (lines in Showtime),connections to be trained are sending only sync symbols at timeinstances when no user data is exchanged. These sync symbols are used toestimate the weights of the crosstalk transfer functions from thejoining lines to the lines in Showtime.

The estimated weights of the crosstalk transfer functions are used forthe rest of the training. According to the recommendation G.995.3 alljoining lines must be trained either completely in parallel or one afteranother. That is, a new training should not be started while anothertraining is already ongoing.

There is a general need for an improved method for initialization of agroup of CPE devices during a training that in part registerscapabilities of the CPE devices.

SUMMARY

A novel procedure is described which provides a method forinitialization of a group of CPE devices (short: CPEs) during a trainingthat in part registers capabilities of the CPEs, wherein at least oneCPE registers late to the training and thus cannot be registered. Inaccordance with the example described herein, the method comprises:determining capabilities of the CPEs during a Joining Phase of thetraining, wherein it is determined whether a CPE device is capable ofemploying vectoring. The method further comprises placing in a holdstatus the at least one CPE that registers late by keeping a line activethat is coupled to the at least one CPE. Another Joining Phase isprovided after the Joining Phase in order to register the at least oneCPE which has registered late.

Furthermore, an apparatus is described which is configured to initializea group of CPEs during a training that in part registers capabilities ofthe CPEs, wherein at least one CPE registers late to the training andthus cannot be registered, and wherein capabilities of the CPEs aredetermined during a joining phase of the training that determineswhether a CPE device is capable of employing vectoring. In accordancewith one example of the invention the apparatus comprises a vectorengine placing the at least one CPE in a hold status that registers lateby keeping a line active that is coupled to the at least one CPE,wherein the vector engine provides another Joining Phase after theJoining Phase to register the at least one CPE that registers late.

BRIEF DESCRIPTION OF THE DRAWINGS

The system may be better understood with reference to the followingdrawings and description. The components in the figures are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention. Moreover, in the figures, likereferenced numerals designate corresponding parts throughout thedifferent views.

FIG. 1 illustrates the various types of crosstalk typically experiencedin a DSL system;

FIG. 2 is a flowchart giving an overview of the ITU-T G.993.5initialization procedure;

FIG. 3 is a flowchart illustrating the mechanism responsible for a CPEmissing the Joining Phase;

FIG. 4 is a flow chart illustrating one improved initialization methodin accordance with one example of the invention and the embedding of themethod in the standardized VDSL2 training including vectoring;

FIG. 5 illustrates a typical sequence of multiple joining phases duringthe start up of a vectoring capable VDSL2 system comprising multiplelines;

FIG. 6 represents an exemplary VDSL2 communication system in accordanceto the present invention,

DETAILED DESCRIPTION

FIG. 1 illustrates the various types of crosstalk typically experiencedin a DSL system. For simplicity, central office (CO) 110 comprises twotransceivers 102, 106 communicating over two subscriber lines to twosets of customer premises equipment (CPE) devices 104, 108. Transceiver102 communicates with CPE device 104, and transceiver 106 communicateswith CPE device 108. As an illustrative example, the crosstalk from COtransceiver 106 and CPE device 108 to either CO transceiver 102 or CPEdevice 104 is described. However, it should be understood thatinterference may also be between the transmitter and receiver on thesame subscriber line in both the upstream and downstream paths, which isthe near-end echo of the transmit signal.

The term “far-end” refers to scenarios in which the source ofinterference is away from the receiving side, and the term “near-end”refers to scenarios in which the source of interference is close to thereceiving side. For example, interference shown by arrow 112 illustratesnoise generated by transceiver 106 coupled into the downstreamcommunications and received by CPE device 104. The term “victim” of“victim user” refers to the line or the circuit being examined forcrosstalk, and the term “disturber” describes the source of thecrosstalk. Since the noise is generated away from the receiving side,this is referred to as downstream far-end crosstalk (FEXT). Likewise,interference shown by arrow 114 illustrates upstream near-end crosstalk(NEXT). Interference shown by arrow 116 illustrates upstream FEXT, andinterference shown by arrow 118 illustrates downstream NEXT. Inparticular, FEXT is a ubiquitous source of noise in VDSL. Accordingly,various needs exist in the industry to address the aforementioneddeficiencies and inadequacies, such as mitigating FEXT.

Vectoring is a transmission method that employs the coordination of linesignals for reduction of crosstalk levels and improvement ofperformance. The degree of improvement depends on the channelcharacteristics. Vectoring may be for a single user or formultiple-users' benefit.

The ITU recommendation G.993.5 covers self-FEXT (far-end crosstalk)cancellation in the downstream and upstream directions. It defines asingle method of self-FEXT cancellation, in which FEXT generated by agroup of near-end transceivers and interfering with the far-endtransceivers of that same group is cancelled. This cancellation takesplace between VDSL2 transceivers, not necessarily of the same profile.The recommendation G.933.5 is intended to be implemented in conjunctionwith ITU-T G.993.2. Multi-pair digital subscriber line (DSL) bonding(see b-ITU-T G.998.1, b-ITU-T G.998.2, and b-ITU-T G.998.3) may beimplemented in conjunction with vectoring.

The techniques described in the recommendation G.993.5 provide means ofreducing self-FEXT generated by the transceivers in a multi-pair cableor cable binder. Self-FEXT cancellation techniques are particularlybeneficial with short cable lengths (<1 km) and limited near-endcrosstalk (NEXT), background noise, and FEXT from systems which are nota part of the vectored group (alien noise). The level of non-self-FEXTnoise sources relative to that of self-FEXT sources determines thedegree to which self-FEXT reduction can improve performance. Anothersignificant factor is the degree to which the self-FEXT cancellingsystem has access to the disturbing pairs of the cable. Maximum gainsare achieved when the self-FEXT cancelling system has access to all ofthe pairs of a cable carrying broadband signals. For multi-bindercables, significant gains are possible when the self-FEXT cancellingsystem has access to all of the pairs of the binder group(s) in which itis deployed and has the ability to cancel at least the majority ofdominant self-FEXT disturbers within the binder. When multiple self-FEXTcancelling systems are deployed in a multi-binder cable without bindermanagement, gains may be significantly reduced.

FIG. 2 is taken from the recommendation G.993.5 describing theinitialization of a connection supporting Vectoring. The initializationprocedure described in in FIG. 2 is based on ITU-T G.993.2initialization with additional steps for FEXT channel estimation. Thefinal mode of vectored operation (i.e., downstream and upstreamvectoring, or downstream only vectoring) is determined during the ITU-TG.994.1 Phase of initialization. FIG. 2 provides an overview of theinitialization procedure for both upstream and downstream directions.For recommendation G.933.5, the ITU-T G.993.2 initialization phases areadopted with some modifications to the SOC messages and addition ofinitialization signals for FEXT channel estimation. The initializationsignals added to the ITU-T G.993.2 Channel Discovery phase and Trainingphase are highlighted in FIG. 2 by bold lines.

If several lines are initialized simultaneously, the initializationprocedures of these lines have to be aligned in time, so that all linespass the vectoring-related phases simultaneously as described in clauses10.3.3.6 and 10.4.3.9 of the recommendation G.933.5.

In the downstream direction, at the beginning of the Channel Discoveryphase, the VTU-O of the initializing line transmits 0-P-VECTOR 1 signalwhich comprises only sync symbols modulated by the pilot sequence andwhich is aligned with sync symbols of vectored lines. The 0-P-VECTOR 1signal allows the VCE (see also FIG. 6) to estimate FEXT channels fromthe initializing lines into the vectored lines. The VCE estimates theseFEXT channels based on the reported clipped error samples from theVTU-Rs of the vectored lines and enables the pre-coding in the VTU-Os ofthese vectored lines to cancel FEXT from the initializing lines intothese vectored lines during the remainder of the initialization of theinitializing lines.

At the beginning of the Training phase, the initializing VTU-O willtransmit O-P-VECTOR 1-1 signal, which is the same as O-P-VECTOR 1 andallows the VCE to update the downstream FEXT channel estimates from theinitializing lines into the vectored lines, prior to transitioning intothe ITU-T G.993.2 Training phase.

After the ITU-T G.993.2 Training phase, the VTU-O transmits theO-P-VECTOR 2 signal, followed by the O-P-VECTOR 2-1 signal, which bothcomprise sync symbols modulated by the pilot sequence and regularsymbols carrying the SOC. During the transmission of O-P-VECTOR 2-1, theVCE estimates FEXT channels from all vectored lines into eachinitializing line and vice versa. Finally, at the end of thetransmission of O-P-VECTOR 2-1, the whole FEXT channel matrix, includingFEXT coefficients from the initializing line into the vectored lines andFEXT coefficients from the vectored lines into each initializing line,is estimated by the VCE. At this point the initialization process iscomplete and the initializing lines may be included in the precodingoperation. After O-P-VECTOR 2-1 transmission is complete, the VTU-O ofthe initializing line enters the Channel Analysis and Exchange phase forestimation of the SNR and determination of the bit loading to be usedduring Showtime.

In the upstream direction, in order to avoid excessive FEXT intovectored lines, the VTU-R of an initializing line, after detection ofthe O-SIGNATURE message in the Channel Discovery phase, startstransmitting an R-P-VECTOR 1 signal, which has the same format asO-P-VECTOR 1. During transmission of the R-P-VECTOR 1, the VCE estimatesthe FEXT channels from the initializing lines into all vectored lines,and enables the VTU-Os of the vectored lines to cancel FEXT from theinitializing lines during the remainder of the initialization of theinitializing lines. The time position of the upstream sync symbols andthe upstream pilot sequence are assigned by the VCE and are indicated tothe VTU-R in the O-SIGNATURE message and by special markers added to theO-P-CHANNEL DISCOVERY V1 signal.

Furthermore, other optional parameters may be added to the O-P-SIGNATUREmessage for upstream transmit power reduction during the initialupstream phase (R-P-VECTOR 1). The upstream transmit power reduction canbe used to reduce the crosstalk of the R-P-VECTOR 1 signals intonon-vectored lines operating in the same binder and provides a flatattenuation of the upstream transmit PSD of R-P-VECTOR 1 in addition tothe standard upstream power back-off as defined in ITU-T G.993.2.

At the beginning of the Training phase, the initializing VTU-R willtransmit the R-P-VECTOR 1-1 signal, which is the same as R-P-VECTOR 1and allows the VCE to update the upstream FEXT channel estimates fromthe initializing lines into the vectored lines, prior to transitioninginto the ITU-T G.993.2 Training phase. The VTU-O transmits theO-P-VECTOR 1-1 signal as a time fill signal while the VTU-R transmitsR-P-VECTOR 1-1.

The initial value of timing advance is assigned by the VTU-O and iscommunicated in O-SIGNATURE, based on the provisional knowledge on thelength of the line. If the timing advance is further re-adjusted duringthe Training phase, then the FEXT channel estimate in the upstreamdirection will be updated at the end of the Training phase to accountfor any resulting change in the FEXT channel (signal R-P-VECTOR 1-2 inFIG. 2). The VTU-O transmits the O-P-VECTOR 2 signal as a time fillsignal while the VTU-R transmits R-P-VECTOR 1-2.

At the end of the Training phase, the VTU-R transmits R-P-VECTOR 2,which comprises Sync symbols modulated by the pilot sequence and regularsymbols carrying the SOC. During the transmission of R-P-VECTOR 2, theVCE estimates the FEXT channels from all vectored lines into theinitializing lines and vice versa. Finally, at the end of the R-P-VECTOR2 transmission, the whole FEXT channel matrix, including FEXTcoefficients from the initializing lines into the vectored lines andFEXT coefficients from vectored lines into the initializing lines, areestimated by the VCE. At this point the initialization process iscomplete and the initializing lines become active members of thevectored group. After R-P-VECTOR 2 transmission is complete, the VTU-Renters the Channel Analysis and Exchange phase for estimation of the SNRand determination of the bit loading to be used during Showtime.

During the transmission of R-P-VECTOR 2, the SOC parameters may be setto provide higher speed SOC, necessary to convey clipped error samplesfrom the VTU-R to the VTU-O. Since both VTU-O and VTU-R already passedthe Training phase, the number of repetitions in the SOC may be reduced(similarly to ITU-T G.993.2 during the Channel Analysis and Exchangephase). This will provide a fast backchannel which is necessary forquick estimation of FEXT channels from vectored lines into theinitializing line.

As mentioned in the introductory part, the weights of the upstream anddownstream crosstalk transfer functions are estimated in the syncsymbols. The data carried in these sync symbols has to be orthogonalfrom line to line. This orthogonality of the sync symbols should not becorrupted in certain phases of an ongoing training by lines which try tosetup a connection after just having finished handshake. To avoidcorruption of the orthogonality in the sync symbols while a vectoringtraining is already ongoing (Joining Phase), the CO stops sending anytone to those CPE devices which try to setup a connection as soon ashandshake has been finished (Quiet Phase) whereupon these CPE devicesinterrupt their connection attempt and thus have to start anothertraining attempt after an undefined time.

The described behavior, which is depicted in FIG. 3, leads to long andunpredictable training times in case that multiple lines want to join avectoring system, i.e. if multiple CPE devices start establishing aconnection with slightly different starting times. FIG. 3 describes thesystem behavior in case that a vectoring training is already ongoing(Joining Group) while an additional CPE device has started to connect(Joining Phase Missed).

In accordance with one example of the invention, lines (i.e. CPEsconnected to the CO via those lines) which attempt to setup a connectionwhile a joining process is already ongoing (and thus could not beconsidered for the current Joining Phase) are collected in the“Prepared-To-Join”-group for the Joining Phase following the ongoingone. Collecting in the “prepared-To-Join” group places the CPE device(s)in a hold status. According to the capability of the vectoring engine(i.e. the vectoring control entity VCE, see FIG. 6) either one,multiple, or even all of the members of the “Prepared-To-Join”-group aretransferred to the Joining Group and trained in parallel as soon as theongoing Joining Phase has been finished. As needed, other lines can,from now on, be again collected in the “Prepared-To-Join”-group for thenext joining phase.

To keep the connection active, the CO sends the “O-P-Pre-VECTOR 1”signal to those CPE devices which are collected in the“Prepared-To-Join”-group. The “O-P-Pre-VECTOR 1” signal shall neithercorrupt the orthogonal sequence of the sync symbols nor the user data ofthe data symbols.

In order to be able to train as many lines as possible in parallel rightat the beginning, the O-P-Pre-VECTOR 1 phase can also be applied to thevery first Joining Phase following the system startup. Following thisprocedure, the training time in case of multi-line joining can besignificantly reduced without impacting the performance of those VDSL2connections, which are already in Showtime. The training of a line whichattempts to setup a connection while a joining phase is already ongoinghas no longer to be interrupted and thus a restart including handshake(see G.994.1) is no longer needed what in case of multiline joiningadditionally leads to a more robust and reproducible training. As COstands for Central Office, it may designate any component of CentralOffice Equipment such as a Digital Subscriber Line Access Multiplexer(DSLAM) or a linecard of a DSLAM. In fact, the vectoring control entitiymay be arranged on a line card used in a DSLAM or in a module of a DSLAMservicing more than one linecard.

The Joining Phases may include a handshake procedure, which is compliantwith the ITU-T G.994.1 standard. The mentioned training (i.e. theTraining Phase) is accomplished in compliance with the ITU-T G.992.3 and993.5 standards. The CPE device(s) of the “Prepared-to-Join” Group areplaced in a hold status by keeping the line(s) active that is coupled tothe respective CPE device(s). Keeping a line active is accomplished bysending a signal to each CPE device, wherein the signal does neitherimpact the joining lines nor the lines (CPE devices) in Showtime. Forexample, the signal includes only Flag Tones in its sync symbols, e.g.when all Pilot Tones are used for the crosstalk adaption. However, incases in which not all Pilot Tones are used for crosstalk adaption, alsothe unused Pilot Tones may be used instead of or in addition to the FlagTones.

The embedding of this procedure in the standardized VDSL2 trainingincluding vectoring is depicted in FIG. 4. As compared to the existingstandard the following extensions of the vectoring training areproposed: (1) definition of the “Prepared-To-Join”-group, (2) definitionof a new state (O-P-Pre-VECTOR 1) between state O-P-QUIET andO-P-VECTOR-1, and (3) definition of the O-P-Pre-VECTOR 1 signal (flagtones only) from the CO to the CPE(s).

Accordingly, the improved procedure described herein provides a methodfor initialization of a group of CPE devices (short: CPEs) during atraining that in part registers capabilities of the CPEs, wherein atleast one CPE registers late to the training and thus cannot beregistered for the reasons described above. In accordance with theexample described herein, the method comprises: determining capabilitiesof the CPEs during a Joining Phase of the training, wherein it isdetermined whether a CPE device is capable of employing vectoring. Themethod further comprises placing in a hold status (i.e. assign to the“Prepared to Join”-group) the at least one CPE that registers late bykeeping a line active that is coupled to the at least one CPE. AnotherJoining Phase is provided after the Joining Phase in order to registerthe at least one CPE which has registered late. The step of placing in ahold status may maintain the line as active by sending signals to the atleast one CPE that indicates that the at least one CPE will beregistered at a later time.

The method described above may be implemented in an apparatus whichincludes a vector engine that is configured to place the at least oneCPE, that registers late, in a hold status (i.e. assign to the “Preparedto Join”-group) by keeping a line active that is coupled to the at leastone CPE. The vector engine provides another Joining Phase after theJoining Phase to register the at least one CPE that registers late.

FIG. 5 depicts a typical sequence of multiple joining phases during thestart up of a vectoring capable VDSL2 system comprising multiple lines.The Acceptance Window n corresponds to a configurable time intervalright after reset. In this time interval all lines which have started tosetup a connection are collected for the first Joining Phase (Line 1 toLine k). The Acceptance Window n+1 is opened for collecting those linesfor the second Joining Phase which have started to setup a connectionwhile Line 1 to Line k are being trained (Line k+1 to Line k+m). In thisexample the last line (Line k+m+1) tries to set-up a connection whilethe second joining group is being trained (Acceptance Window n+2). Thetimings outlined FIG. 5 are for controlling the size of the AcceptanceWindows.

In the timing diagrams of FIG. 5 T_(Dmin) is the minimum time forO-P-Pre-VECTOR-1. This configurable time interval Is re-started withevery “GHS_COMPLETE” message Issued In Acceptance Window n. T_(Dmax)denotes the maximum time for O-P-Pre-VECTOR-1. Lines finishing G.hsafter this configurable time interval are neglected for the currentjoining Phase. Lines which terminate G.hs within T_(Dmax) after theprevious line has reached O-P-Pre-VECTOR-1 and within T_(Dmax) after thefirst line of the current Joining Phase has reached O-P-Pre-VECTOR-1 areaccepted for the current Joining Phase (Acceptance Window n). The timeTSAmin is the minimum time for xTalk adaptation in Showtime. This timeinterval is hard-coded in the firmware. T_(SAplus) is an additional timefor xTalk adaptation In Showtime. This configurable time interval Is forfurther fine-tuning of the xTalk coefficients. Lines which terminateG.hs within T_(SAmin)+T_(SAplus) after the lines of Acceptance Window nhave reached Showtime are accepted for the next Joining Phase(Acceptance Window n+1).

FIG. 6 illustrates a DSLAM (Digital Subscriber Line Access Multiplexer)100 located at a CO (central office) or at a remote location closer tosubscriber premises, and comprising G.993.5 compliant transceiver units101 (or VTUC1), 102 (or VTUC2) and 103 (or VTUC3), a line initializationcontroller 111 (or CTRL), and the above-mentioned vectoring engine orvectoring control entity 112 (or VCE). The line initializationcontroller 111 as we as the vectoring control entity 112 are coupled tothe transceivers units 101, 102 and 103. Regardless of the actuallocation of the DLSAM it is regarded as part of Central Office Equipment(COE).

The DSLAM 100 is coupled to CPE devices via Unshielded Twisted Pairs(UTP), such as CATS cables. The first transceiver unit 101 is coupled toa first remote transceiver unit 201 (or VTUR1) via a first subscriberline L1; the second transceiver unit 102 is coupled to a second remotetransceiver unit 202 (or VTUR2) via a second subscriber line L2; and thethird transceiver unit 103 is coupled to a third remote transceiver unit203 (or VTUR3) via a third subscriber line L3. The remote transceiverunits 201, 202 and 203 form part of e.g. a modem, a gateway, a router, aset top box, a laptop, etc.

The subscriber lines L1, L2 and L3 are bundled in a cable binder 301together with further subscriber lines, and induce crosstalk into eachother as they are in close vicinity over whole or part of their length.In DMT based DSL systems, crosstalk mostly reduces to FEXT (Far-EndCrosstalk); some substantial amount of the signal transmitted by atransceiver unit (the disturber) couples into a neighboring line andimpairs reception of the direct signal transmitted over that neighboringline at a remote transceiver unit (the victim). For instance, thedownstream signal transmitted by the VTUC 101 over line L1 couples intoline L2 and is detected as noise by the VTUR 202. Also, the upstreamsignal transmitted by the VTUR 203 over line L3 couples into line L1 andis detected as noise by the VTUC 101.

The DSLAM 100 further includes a precoder to mitigate FEXT in downstreamdirection and/or a FEXT canceller to mitigate FEXT in upstreamdirection. Presently, the subscriber lines L1, L2 and L3 form part ofthe same vectoring group, which may comprises further subscriber lines,and the precoder and/or FEXT canceller are configured to mitigatedownstream and/or upstream crosstalk between lines of the vectoringgroup.

Typically, the frequency samples of each downstream data symbol of eachsubscriber line are forwarded to the precoder by the transceiver units,and crosstalk-compensated samples are returned by the precoder to thetransceiver units for Inverse Discrete Fourier Transform (IDFT), Digitalto Analog Conversion (DAC) and further transmission over the subscriberline. Similarly, the frequency samples of each received upstream datasymbol are forwarded to the crosstalk canceller by each transceiverunit, and (almost) crosstalk-free samples are returned by the crosstalkcanceller to each transceiver unit for detection and demodulation.

Primarily, the transceiver units 101, 102 and 103 are configured toterminate the subscriber lines L1, L2 and L3 respectively, and toinitialize and operate DSL communication channels CH1, CH2 and CH3,respectively. So are the remote transceiver units 201, 202 and 203.

The transceiver units 101, 102 and 103 are further configured to notifythe line initialization controller 111 about a new line starting up(joining line), being on behalf of the transceiver unit at the centraloffice or the transceiver unit at the customer premises, and further tocarry out the DSL initialization procedure after approval from the lineinitialization controller 111.

The communication channels CH1, CH2 and CH3 comprise a downstream datacommunication path and an upstream data communication path usingdistinct downstream and upstream frequency bands (frequency divisionmultiplexing). Respective bit loadings and gains for downstream andupstream carriers are determined and agreed upon during lineinitialization, thereby yielding a total downstream data rate and atotal upstream data rate.

The DSL initialization procedure comprises a handshake phase, multiplecrosstalk acquisition phases, a channel discovery phase, a channeltraining phase, and a channel analysis and exchange phase.

The handshake phase is described in G.994.1, and makes use of one ormore predefined set of carriers (so-called signaling family) dependingon the one or more specific annex of the recommendation being supported.Those pre-defined carrier sets comprise very few carriers only(typically 2 or 3), thereby causing little interference on neighboringlines.

The handshake procedure comprises a first sub-phase A during which peertransceiver units acknowledge their mutual presence by exchanging probesignals comprising a carrier set and acquire clock synchronization forthe probe signals, and a second sub-phase B during which peertransceiver units exchange their respective capabilities and agree on acommon mode for training and operation. A successful completion of thehandshake phase will lead to the first crosstalk acquisition phaseO-P-VECTOR 1.

All messages in the handshake phase are sent with the one or morelimited set of carriers. All carrier frequencies within a carrier set,and all carrier sets, are simultaneously modulated with the same databits using Differential Phase Shift Keying (DPSK). The transmit point isrotated 180° from the previous point if the transmit bit is a 1, and thetransmit point is left unchanged if the transmit bit is a 0.

Initially, the VTUR is in state R-SILENT0 transmitting silence, and theVTUC is in state C-SILENT1 transmitting silence.

For duplex mode of operation, and in the event of the VTUR initiatingthe initialization procedure, the first handshake sub-phase A startswith the VTUR transmitting a R-TONES-REQ signal from one or both of itssignaling family with phase reversals every 16 ms. When this has beendetected by the VTUC, the VTUC shall respond by transmitting a C-TONESsignal from one or both of its signaling family. When this has beendetected by the VTUR, the VTUR shall transmit silence (R-SILENT1) for 50to 500 ms and shall then transmit a RTONE1 signal from only onesignaling family. When the VTUC has detected RTONE1 signal, it shallrespond by transmitting GALF (0x81=one complement of 0x7E) on modulatedcarriers (C-GALF1). When the VTUR has detected GALF, it shall respond bytransmitting FLAGs (0x7E) on modulated carriers (RFLAG1). When the VTUChas detected FLAGS, it shall respond by transmitting FLAGs (C-FLAG1).When the VTUR has detected FLAGS, it shall enter the sub-phase B byinitiating the first message transaction.

In the event of the VTUC initiating the initialization procedure, thefirst handshake sub-phase A starts with the VTUC directly transmittingC-TONES and keeps on as aforementioned.

Slightly different timing and signals are defined for half-duplex modeof operation.

The second handshake sub-phase B starts with the VTUR sending aCAPABILITY LIST REQUEST CLR message conveying the capabilities of theVTUR (that is to say, a list of possible modes of operation), andwhereby the VTUR further requests the VTUC capabilities. The VTUCreplies with a CAPABILITY LIST CL message conveying the VTUCcapabilities. The VTUR acknowledges the good receipt of the CL messageby returning an ACK(1) acknowledgment.

The sub-phase B carries on by either the VTUR or the VTUC selecting acommon mode of operation according to the advertised capabilities. Thisis achieved by issuing a MODE SELECT MS message conveying the selectedmode of operation, and by returning an ACK(1) acknowledgment. Typically,the VTUR selects the most appropriate mode of operation at once andissues the MS message. Yet, the VTUR can request the VTUC to select aparticular mode of operation by issuing a MODE REQUEST MR message, orcan propose a particular mode of operation while leaving the finaldecision to the VTUC by issuing a MODE PROPOSAL MP message. Once aparticular mode of operation is acknowledged, the VTUC and the VTURenter the O-P-VECTOR 1 and R-P-VECTOR 1 crosstalk acquisition phasesrespectively.

G.994.1 defines a provision for re-iterating through the sub-phase B byallowing the VTUC to respond to the MS message with a REQUEST-CAPABILITYLIST REQUEST REQ-CLR message requesting the VTUR to proceed again withCLR/CL/ACK(1) message exchange and further MS/MR/MP/ACK(1) messageexchange, and without returning to the initial transaction state(R-SILENT0).

The line initialization controller 111 is further configured to controlthe DSL initialization procedure over each subscriber line, and morespecifically to supply the VTUCs 101, 102 and 103 with a number n ofiterations for the second handshake sub-phase B to be carried out, aswell as with an additional delay D for answering any message requiring aspecific response or acknowledgment from the VTUC during the sub-phaseB, such as a CLR or MS or MR or MP message.

The VTUCs 101, 102 and 103 are further configured to measure theexecution times TA and TB of the first and second handshake sub-phases Aand B respectively (not including the configured additional delay D, ifany), and to report the so-measured execution time to the lineinitialization controller 111. By so doing, the execution time spreadfrom different CPE manufacturers and/or implementations is accountedfor.

Although the initialization controller 111 has been depicted as acentral unit within the access node 100, it can be partly or whollydistributed across the VTUCs 101, 102 and 103.

Exemplary implementations discussed herein may have various componentscollocated; however, it is to be appreciated that the various componentsof the arrangement may be located at distant portions of a distributednetwork, such as a communications network and/or the Internet, or withina dedicated secure, unsecured and/or encrypted arrangement. Thus, itshould be appreciated that the components of the arrangements may becombined into one or more apparatuses, such as a modem, or collocated ona particular node of a distributed network, such as a telecommunicationsnetwork. Moreover, it should be understood that the components of thedescribed arrangements may be arranged at any location within adistributed network without affecting the operation of the arrangements.For example, the various components can be located in a Central Officemodem (CO, ATU-C, VTU-0), a Customer Premises modem (CPE, ATU-R, VTU-R),an xDSL management device, or some combination thereof. Similarly, oneor more functional portions of the arrangement may be distributedbetween a modem and an associated computing device.

The above-described arrangements, apparatuses and methods may beimplemented in a software module, a software and/or hardware testingmodule, a telecommunications test device, a DSL modem, an ADSL modem, anxDSL modem, a VDSL modem, a linecard, a G.hn transceiver, a MoCA®transceiver, a Homeplug transceiver, a powerline modem, a wired orwireless modem, test equipment, a multicarrier transceiver, a wiredand/or wireless wide/local area network system, a satellitecommunication system, network-based communication systems, such as anIP, Ethernet or ATM system, a modem equipped with diagnosticcapabilities, or the like, or on a separate programmed general purposecomputer having a communications device or in conjunction with any ofthe following communications protocols: CDSL, ADSL2, ADSL2+, VDSL1,VDSL2, HDSL, DSL Lite, IDSL, RADSL, SDSL, UDSL, MoCA, G.hh, Homeplug orthe like.

Additionally, the arrangements, procedures and protocols of thedescribed implementations may be implemented on a special purposecomputer, a programmed microprocessor or microcontroller and peripheralintegrated circuit element(s), an ASIC or other integrated circuit, adigital signal processor, a flashable device, a hard-wired electronic orlogic circuit such as discrete element circuit, a programmable logicdevice such as PLD, PLA, FPGA, PAL, a modem, a transmitter/receiver, anycomparable device, or the like. In general, any apparatus capable ofimplementing a state machine that is in turn capable of implementing themethodology described and illustrated herein may be used to implementthe various communication methods, protocols and techniques according tothe implementations.

Furthermore, the disclosed procedures may be readily implemented insoftware using object or object-oriented software developmentenvironments that provide portable source code that can be used on avariety of computer or workstation platforms. Alternatively, thedisclosed arrangements may be implemented partially or fully in hardwareusing standard logic circuits or VLSI design. The communicationarrangements, procedures and protocols described and illustrated hereinmay be readily implemented in hardware and/or software using any knownor later developed systems or structures, devices and/or software bythose of ordinary skill in the applicable art from the functionaldescription provided herein and with a general basic knowledge of thecomputer and telecommunications arts.

Moreover, the disclosed procedures may be readily implemented insoftware that can be stored on a computer-readable storage medium,executed on programmed general-purpose computer with the cooperation ofa controller and memory, a special purpose computer, a microprocessor,or the like. In these instances, the arrangements and procedures of thedescribed implementations may be implemented as program embedded onpersonal computer such as an applet, JAVA® or CGI script, as a resourceresiding on a server or computer workstation, as a routine embedded in adedicated communication arrangement or arrangement component, or thelike. The arrangements may also be implemented by physicallyincorporating the arrangements and/or procedures into a software and/orhardware system, such as the hardware and software systems of atest/modeling device.

The implementations herein are described in terms of exemplaryembodiments. However, it should be appreciated that individual aspectsof the implantations may be separately claimed and one or more of thefeatures of the various embodiments may be combined.

1. A method for initialization of a group of customer premises equipmentdevices (CPEs) during a training that in part registers capabilities ofthe CPEs, wherein at least one CPE registers late to the training andcannot be registered; the method comprising: determining capabilities ofthe CPEs during a joining phase of the training, wherein it isdetermined whether a CPE device is capable of employing vectoring;placing in a hold status the at least one CPE that registers late bykeeping a line active that is coupled to the at least one CPE; andproviding another joining phase after the joining phase to register theat least one CPE that registers late.
 2. The method according to claim1, further wherein the step of placing in a hold status maintains theline as active by sending signals to the at least one CPE that indicatesthat the at least one CPE will be registered at a later time.
 3. Themethod of claim 1 wherein the Joining Phase includes a handshakeprocedure compliant with the ITU-T G.994.1 standard.
 4. The method ofclaim 1 wherein the training includes a Training Phase compliant withthe ITU-T G.992.3 and 993.5 standards.
 5. The method of claim 1 whereindetermining capabilities includes: determining whether the at least oneCPE supports being placed in a hold status.
 6. The method of claim 1wherein keeping a line active that is coupled to the at least one CPEincludes: sending a signal to the CPE which does neither impact thejoining lines nor the lines in Showtime.
 7. The method of claim 6,wherein the signal, which does neither impact the joining lines nor thelines in Showtime, includes only Flag Tones.
 8. The method according toclaim 1, wherein the other joining phase is provided directly after thejoining phase.
 9. The method according to claim 1, wherein capabilitiesto be determined include capabilities as defined in ITU-T recommendationG.992.1.
 10. The method according to claim 9, wherein the capabilitiesinclude at least echo cancellation capabilities.
 11. An apparatus forinitialization of a group of customer premises equipment devices (CPEs)during a training that in part registers capabilities of the CPEs,wherein at least one CPE registers late to the training and cannot beregistered, wherein capabilities of the CPEs are determined during ajoining phase of the training that determines whether a CPE device iscapable of employing vectoring, the apparatus comprising: a vectorengine placing the at least one CPE in a hold status that registers lateby keeping a line active that is coupled to the at least one CPE; andwherein the vector engine provides another joining phase after thejoining phase to register the at least one CPE that registers late. 12.The apparatus according to claim 11, wherein the vector engine placesthe at least one CPE in a hold status by maintaining the line as activeby sending signals to the at least one CPE that indicates that the atleast one CPE will be registered at a later time.
 13. The apparatusaccording to claim 11, wherein the vector engine is a vectoring controlentity (VCE) located within an Access Node of a Digital Subscriber LineAccess Multiplexer (DLSAM).
 14. The apparatus according to claim 11,wherein the Joining Phase includes a handshake procedure in accordancewith the ITU-T G.994.1 standard.
 15. The apparatus according to claim11, wherein the determined capabilities include the capability of the atleast one CPE to be placed in a hold status.
 16. The apparatus accordingto claim 11, wherein capabilities to be determined include capabilitiesas defined in ITU-T recommendation G.992.1.
 17. The apparatus accordingto claim 16 wherein the capabilities include at least echo cancellationcapabilities.
 18. The apparatus according to claim 11, wherein keeping aline active includes: sending a signal to the CPE which does neitherimpact the joining lines nor the lines in Showtime.
 19. The apparatusaccording to claim 18, wherein the signal, which does neither impact thejoining lines nor the lines in Showtime, includes only Flag Tones. 20.The apparatus according to claim 11, wherein the other joining phase isprovided directly after the joining phase.
 21. A linecard for use in aDSLAM and configured to initialize a group of customer premisesequipment devices (CPEs) during a Training Phase in which capabilitiesof the CPEs are registered, wherein at least one CPE registers late tothe training and cannot be registered, wherein capabilities of the CPEsare determined during a Joining Phase of the Training Phase fordetermining whether a CPE device is capable of employing vectoring, thelinecard comprising: a vector engine configured to place the at leastone CPE, that registers late, in a hold status by keeping a line activethat is coupled to the at least one CPE; and wherein the vector engineprovides another joining phase after the joining phase to register theat least one CPE that registers late.